In SiC (silicon carbide), a high-temperature type (α-type) having a hexagonal crystal structure and a low-temperature type (β-type) having a cubic crystal structure are known. SiC is characterized, in comparison with Si, by having high thermal resistance, a broad band gap, and a high dielectric breakdown field strength. For that reason, a semiconductor including an SiC single crystal is expected as a candidate material of a next-generation power device substituting for an Si semiconductor. In particular, α-type SiC has a band gap broader than β-type SiC and hence the α-type SiC attracts attention as a semiconductor material of an ultralow power-loss power device.
α-type SiC has, as the principal crystal plane, a {0001} plane (hereunder referred to also as “c-plane”), and a {1-100} plane and a {11-20} plane (hereunder referred to also as “a-plane” collectively) perpendicular to the {0001} plane.
A c-plane growth method and an a-plane growth method have heretofore been known as methods of obtaining an α-type SiC single crystal. The “c-plane growth method” cited here means a method of: using as a seed crystal an SiC single crystal in which a c-plane or a plane having an offset angle to the c-plane in a prescribed range is exposed as a growth plane; and growing an SiC single crystal over the growth plane by a sublimation reprecipitation method or the like. Meanwhile, the “a-plane growth method” means a method of: using as a seed crystal an SiC single crystal in which an a-plane or a plane having an offset angle to the a-plane in a prescribed range is exposed as a growth plane; and growing an SiC single crystal over the growth plane.
In order to materialize a high-performance SiC power device, reducing a leak current and inhibiting withstand voltage from deteriorating in an SiC device are indispensable requirements (Non-patent Literature 1), and it is necessary to reduce a dislocation density in an SiC single crystal that causes the drawbacks.
As dislocations existing in an SiC single crystal, there are a micropipe, a threading screw dislocation, a basal plane dislocation, and a threading edge dislocation. Among them, a micropipe is now being eradicated by the progress of technologies for improving the quality of an SiC single crystal and hence a threading screw dislocation, a basal plane dislocation, and a threading edge dislocation are going to be the next targets of the improvement. Among those three types of dislocations, each of the most of the basal plane dislocations and the threading edge dislocations has a Burgers vector (vector representing the orientation of the inconsistency of atoms around a dislocation line) in a {0001} in-plane direction. Consequently, such dislocations are dislocations capable of changing the orientations and convertible to each other in a crystal while the way of the distortion of crystal lattices is maintained. As a result in general, only by reducing either the basal plane dislocations or the threading edge dislocations, the reduction of one type dislocations causes the other type dislocations to increase.
This is backed by Patent Literature 1, too. The literature describes that, when a single crystal grows on a wafer having a basal plane dislocation density of 10,000 pieces/cm2 and an edge dislocation density of 10,000 pieces/cm2, the basal plane dislocation density comes to be 500 pieces/cm2 and the edge dislocation density comes to be 19,500 pieces/cm2 and thus the edge dislocations increase in exchange of the reduction of the basal plane dislocations. Consequently, it is difficult to manufacture a high-performance SiC device by using such a crystal.
The crystal described in the literature is obtained not by a gas phase method used in the present invention but by a solvent epitaxy method (liquid phase method). Meanwhile, it is known that a dislocation line changes the orientation while a Burgers vector is conserved (Burgers vector conservation law). By the Burgers vector conservation law, the principle that an edge dislocation density increases in exchange of the reduction of a basal plane dislocation density does not change regardless of a manufacturing method such as a gas phase method or a liquid phase method.
Although it is frequently attempted recently to convert basal plane dislocations to threading edge dislocations during epitaxial growth in particular in order to inhibit the forward direction degradation of a bipolar device (Non-patent Literature 2), an increase in the threading edge dislocations as stated above eventually causes leak current to increase (Non-patent Literature 1).
The present inventors: have cut out a crystal obtained by a method of carrying out a c-plane growth after a-plane growth is repeated, a so-called repeated a-face (a-plane) growth (RAF) method (Patent Literature 2), along a c-plane and a plane vertical to the c-plane; and have analyzed a three-dimensional structure of dislocations from X-ray topography images thereof (Non-patent Literature 3). The major reason why a dislocation structure has been able to be analyzed is that a dislocation density has reduced in a crystal by the RAF method and a clearer dislocation image has been obtained in comparison with conventional SiC. In an SiC single crystal for a power device however, the densities of the basal plane dislocations and the threading edge dislocations have been still high.